The present invention relates to a piezoelectric ceramic body having at least one silver-containing internal electrode.
Ceramic piezoactuators are widely known that are composed of a multiplicity of thin, ceramic piezoactive layers, each of which being separated from the others by an internal electrode layer, via which they can be electrically contacted and driven (see, e.g., German Patent Application No. 199 51 016.4).
Piezoactuators of this type are essentially composed, for example, of a PZT ceramic, i.e., Pb(TixZr1-x)O3 where 0.4 less than x less than 0.6 in the form of a multiplicity of ceramic piezoactive layers made of this PZT ceramic, sintered together, which can be electrically driven in a defined manner by internal electrodes, mounted in between, and which, as a result of the inverse piezoeffect, undergo an expansion or compression in response to the application of an external voltage. In typical driving voltages of 100 to 300 V, this alteration in length is in the range of 0.1 to 0.3%.
The internal electrodes in piezoelectric ceramic bodies must, on the one hand, be made of materials whose melting point is higher than the temperatures necessary for sintering the ceramics. In addition, the materials of the internal electrodes must be oxidation-stable, because the sintering of the insulating layers acting as actuators, and therefore of the entire piezoelectric ceramic body is carried out in the presence of oxygen or air. In order to assure this oxidation-stability, rare metals or rare-metal alloys are usually used as the materials for the internal electrodes.
One internal-electrode material frequently used is platinum, which, however, is very expensive, so that silver-palladium alloys are often substituted, which have a typical composition of 70 percentage by mass silver and 30 percentage by mass palladium. This composition assures a melting point that is sufficiently high for the sintering.
One disadvantage in using silver in the internal electrodes is the fact that, during sintering in a co-firing process, the result can be a diffusion of silver from the electrodes into the neighboring insulating layers. This leads to an undesirable influencing of the electromechanical properties of the piezoelectric ceramic body, because the diffusing silver is incorporated, for example, on the Pb locations of the PZT piezoceramic. A further disadvantage in this undesirable diffusion is that when large quantities of silver are diffused into the PZT ceramic, after a silver saturation has been reached, silver secondary phases form, thus reducing the specific electrical resistance of the otherwise electrically insulating PZT material. The result can be undesirable electrical breakdowns.
Finally, a disadvantage of the known silver-palladium alloys as the internal-electrode material is that the proportion of palladium at approximately 30 percent by mass is still relatively high, which also makes this internal-electrode material relatively expensive. However, an undesirable reduction in the palladium content results in a further increase in the silver content, which even further increases the aforementioned disadvantages due to the undesirable diffusion effects.
In contrast to the related art, the piezoelectric ceramic body according to the present invention has the advantage that a diffusion of silver from the internal electrodes into the neighboring insulating layers is reduced or suppressed. Thus the silver added to the internal-electrode material essentially remains in the electrode, so that the negative influencing of the properties of the piezoelectric ceramic material, for example a PZT ceramic, by the diffusion of the silver is significantly reduced.
It is especially advantageous if the component of the internal-electrode material reducing or inhibiting the diffusion of silver is a ceramic component, especially a piezoelectric ceramic component such as PZT ceramic. In this way, it is possible to adapt in a simple manner the mechanical properties of the material of the internal electrode, especially with regard to temperature behavior and thermal expansion, to the neighboring insulating layers, which usually are made of PZT ceramics. Furthermore, in this manner, the adhesion between the internal electrodes and the neighboring insulating layers is improved.
Altogether, a reduction in the production of waste in the manufacturing process is therefore brought about, because the above-mentioned effects suppress the occurrence of delaminations or cracks in the generated piezoelectric ceramic body during and after the sintering.
In addition, it is advantageous if the proportion of the component reducing or inhibiting the diffusion of silver does not exceed 50 percent by volume, with respect to the entire volume of the material of the internal electrode. In this manner, it is assured that a sufficiently high electrical conductivity of the internal electrodes is always maintained.
The optimal value for the proportion of the component reducing or inhibiting the diffusion of silver, for example the added PZT ceramic, can be established by the worker skilled in the art for an individual case on the basis of simple preliminary tests, because this proportion is highly dependent on the particle sizes of the metallic and ceramic powders used for manufacturing the insulating layers and the internal electrodes.
Furthermore, it has proven to be very advantageous if the component of the internal-electrode material reducing or inhibiting the diffusion of silver also contains rare-earth metals such as lanthanum or neodymium, subgroup elements such as niobium, alkali metals such as lithium, sodium, or potassium, or alkaline-earth metals such as strontium. These metals have a favorable influence on the characteristic physical sizes of the piezoelectric multilayer actuator generated and can be added both to the insulating layers as well as to the material of the internal electrodes.
The rare-earth metals, the subgroup elements, the alkali metals, and/or the alkaline-earth metals are preferably added to the material of the insulating layers in the form of a dopant having a content of less than 8 mol %, preferably less than 5 mol %, with respect to the material of the internal electrode and of the insulating layer.
Finally, the added components, reducing or inhibiting the diffusion of silver, are not limited to piezoelectric layers made of a PZT-type material.